The conditional stimulus effects of nicotine vary as a function of training dose

Use of the overexpectation effect to reduce conditioned seeking behavior controlled by nicotine

Nicotine produces robust stimulus effects that can be conditioned to form associations with reinforcing nondrug stimuli. We examine how established associations to the nicotine stimulus may be weakened via the overexpectation effect. In two experiments, we separately conditioned sucrose associations to the interoceptive nicotine stimulus (0.4 mg/kg, SC) and to a "noisy" exteroceptive contextual stimulus (oscillating houselight and white noise) via the discriminated goal-tracking task. Thereafter, we presented additional sucrose pairings with the nicotine and noisy stimuli, now in compound. Testing of the conditioned goal-tracking evoked by the nicotine and noisy stimuli in isolation-before versus after compound conditioning (Experiment 1) or between treatment and control groups (Experiment 2)-demonstrated an attenuation of conditioned responding via the overexpectation effect. We suggest that applications of the overexpectation effect may provide some promise for treatments seeking to attenuate drug-evoked conditioned responses in situations where extinction-based interventions are not suitable.

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Varenicline serves as the training stimulus in the drug-discriminated goal-tracking task with rats: initial evaluation of potential neuropharmacological processes

Varenicline (Chantix) is an FDA-approved smoking cessation aid that is pharmacologically similar to nicotine, the primary addictive component found within tobacco. In support of this similarity, previous drug discrimination research in rats has reported that the internal or interoceptive stimulus effects of nicotine and varenicline share stimulus elements. Those shared elements appear to be mediated, in part, by overlapping action at alpha4beta2-containing nicotinic acetylcholine receptors (nAChRs). The research supporting this conclusion, however, has only used nicotine, and not varenicline, as the training drug. Accordingly, we used the discriminated goal tracking (DGT) task in which 1 mg/kg varenicline signaled intermittent access to sucrose. On separate intermixed saline days, sucrose was not available. Rats acquired the discrimination as measured by a differential increase in dipper entries (goal tracking) evoked by varenicline. These rats then received a series of tests with several doses of varenicline, nicotine, nornicotine (a metabolite of nicotine and tobacco alkaloid), sazetidine-A (a partial alpha4beta2 agonist), PHA-543613 (an alpha7 agonist), and bupropion (a norepinephrine and dopamine reuptake inhibitor). Control of goal tracking by varenicline was dose-dependent. Nicotine and nornicotine evoked responding comparable to the varenicline training dose indicating full substitution. Sazetidine-A partially substituted for the varenicline stimulus, whereas bupropion and PHA-543613 evoked little to no varenicline-like responding. These findings indicate that varenicline can serve as the training stimulus in the DGT task. Further, stimulus control of varenicline in the DGT task is driven by its partial agonist activity at alpha4beta2-containing nAChRs. The use of this approach could lead to a better understanding of the pharmacological action of varenicline and help guide treatment geared towards tobacco cessation through a more targeted development of novel synthetically designed, subunit-specific pharmacological interventions.

Nicotine chronic tolerance development and withdrawal in the planaria (Schmidtea mediterranea)

Chronic nicotine exposure reduces sensitivity to the effects of nicotine, which then results in behavioural changes and tolerance development. In the planaria, a valuable first-stage preclinical model for addictive behaviour, acute nicotine administration has been shown to steadily alter the motility of the animals, a result that has been interpreted as evidence of tolerance and withdrawal effects; however, chronic exposure - typically regarded as a condition for the development of tolerance - and the role of the contextual cues have not been systematically assessed. The present study assessed the acute and chronic effects of nicotine on the motility of planarians (Schmidtea mediterranea). The animals in the experimental groups received long chronic exposure to nicotine (ten daily 30 min exposures); a control group was exposed to water in the same context but in the absence of the drug. The motility of the animals was closely monitored on every exposure. Following this phase, all the animals were subject to three different tests: in the presence of the exposure context (without the drug, Test 1); in the presence of nicotine in the exposure context (Test 2); and in the presence of the drug in a novel context (Test 3). Exposure to nicotine consistently reduced motility; the motility in the presence of nicotine increased with repeated exposures to the drug, an instance of tolerance development. Tolerance development was dependent on nicotinic receptor activation, because it was blocked by the co-administration of mecamylamine. However, this tolerance was found to be independent of the contextual cues where the effects of the drug had been experienced. The results are discussed by reference to the existent theories of tolerance development to drugs.

Effects of nicotine conditioning history on alcohol and methamphetamine self-administration in rats

BACKGROUND

Smoking constitutes a significant public health risk. Alcohol and methamphetamine use disorders are also highly co-morbid with smoking, further increasing negative health outcomes. An important question in determining the underlying neurobiology of nicotine poly-drug use is understanding whether having a positive history with nicotine effects alters later drug-taking behavior.

METHODS

The current experiments sought to elucidate whether having an appetitive nicotine conditioning history would affect later alcohol or methamphetamine self-administration. Adult male and female Long-Evans rats were first trained on a discriminated goal-tracking task in which the interoceptive effects of nicotine predicted sucrose reinforcement. As a control, pseudo-conditioned groups were included that had equated nicotine and sucrose experience. Rats were then shifted to either alcohol self-administration or methamphetamine self-administration.

RESULTS

Nicotine conditioning history had no effect on acquisition or maintenance of alcohol self-administration in males or females. In contrast, an appetitive nicotine conditioning history decreased methamphetamine self-administration in female rats, but not males.

CONCLUSIONS

In female, but not male, rats, an appetitive conditioning history with nicotine decreases methamphetamine, but not alcohol, self-administration. This dissociation suggests that the effects may be due to a specific increase in the reinforcing value of methamphetamine. This may have implications for better understanding the progression of drug use from nicotine to methamphetamine.

Cocaine preexposure enhances sexual conditioning and increases resistance to extinction in male Japanese quail

The incentive-sensitization theory posits that drug addiction results from altered learning and motivational processes that stem from drug-induced changes in the brain's reward circuitry. Although it is generally accepted that problematic drug use results from these neuroadaptations, less research has focused on how these neural changes affect the incentive-motivational properties of naturally rewarding stimuli such as sex. The present set of experiments was conducted to investigate (1) dose-dependent effects of preexposure to chronic cocaine on sexual conditioning and (2) how prior cocaine exposure affects the extinction of sexually conditioned behavior in male Japanese quail. In Experiment 1, male quail were exposed to saline or to cocaine (5, 10, or 20 mg/kg ip) for 10 days, and their locomotor activity was measured. After a washout period, ten sexual-conditioning trials were conducted in which a light (CS) was presented prior to the presentation of a female quail (US) and approach to the light was measured. The results showed that cocaine dose-dependently enhanced sexually conditioned approach behavior and copulation. Experiment 2 was procedurally similar to Experiment 1, except that the quail received either saline or 10 mg/kg cocaine (ip) and 24 extinction trials were conducted. During extinction, no female was presented after the CS. The results showed that preexposure to cocaine delayed extinction. Therefore, cocaine may dose-dependently increase the strength of sexual conditioning, and this may subsequently increase resistance to extinction. These findings and their possible mechanisms are explored.

The effect of switching pharmacological intervention during extinction on nicotine-evoked conditioned responding in rats

RATIONALE

Pharmacotherapies are often utilized to aid in smoking cessation, and switching medication when treating nicotine dependence has become more commonplace. Although common, little is known about the impact of the initial therapy on the effects of the subsequent therapy.

OBJECTIVES

To begin to fill this gap in our understanding, this project determined how switching compounds that share stimulus elements with nicotine during extinction altered extinction responding and generalization of this extinction back to nicotine.

METHODS

Rats were trained in a discriminated goal-tracking task where nicotine administration was followed by intermittent sucrose access; sucrose was withheld following saline administration. In experiment 1, nornicotine supplanted nicotine in extinction sessions 1-3 and then a switch to varenicline on extinction sessions 4-6 was examined. In experiment 2, the reverse was investigated; varenicline to start extinction and then a switch to nornicotine. Generalization of extinction back to the nicotine stimulus was then assessed by generating a cumulative dose-effect curve.

RESULTS

Generalization of extinction back to the training nicotine stimulus was greater if nornicotine had been received at any point in extinction compared to only receiving varenicline. Whereas, extinction with varenicline alone showed more generalization to lower doses of nicotine.

CONCLUSIONS

A switch in cessation pharmacotherapy during extinction did not impede or enhance generalization back to the nicotine-training stimulus. The nornicotine stimulus appears to share more stimulus overlap with the 0.4 mg/kg nicotine stimulus and varenicline may share more overlap with lower nicotine doses.

Interoceptive conditioning with nicotine using extinction and re-extinction to assess stimulus similarity with bupropion

Bupropion is an atypical antidepressant that increases long-term quit rates of tobacco smokers. A better understanding of the relation between nicotine and this first-line medication may provide insight into improving treatment. For all experiments, rats first had nicotine (0.4 mg base/kg) and saline session intermixed; intermittent access to sucrose only occurred on nicotine session. Nicotine in this protocol comes to differentially control "anticipatory" dipper entries. To more closely examine the overlap in the interoceptive stimulus effects of nicotine and bupropion, we assessed whether subsequent prolonged and repeated non-reinforced (extinction) sessions with the bupropion stimulus could weaken responding to nicotine (i.e., transfer of extinction). We also examined whether retraining the discrimination after initial extinction and then conducting extinction again (i.e., re-extinction) with bupropion would affect responding. We found that bupropion (20 and 30 mg/kg) fully substituted for the nicotine stimulus in repeated 20-min extinction sessions. The extent of substitution in extinction did not necessarily predict performance in the transfer test (e.g., nicotine responding unchanged after extinction with 20 mg/kg bupropion). Generalization of extinction back to nicotine was not seen with 20 mg/kg bupropion even after increasing the number of extinction session from 6 to 24. Finally, there was evidence that learning in the initial extinction phase was retained in the re-extinction phase for nicotine and bupropion. These findings indicate that learning involving the nicotine stimuli are complex and that assessment approach for stimulus similarity changes conclusions regarding substitution by bupropion. Further research will be needed to identify whether such differences may be related to different facets of nicotine dependence and/or its treatment.

Iptakalim attenuates self-administration and acquired goal-tracking behavior controlled by nicotine

Iptakalim is an ATP-sensitive potassium channel opener, as well as an α4β2-containing nicotinic acetylcholine receptor (nAChR) antagonist. Pretreatment with iptakalim diminishes nicotine-induced dopamine (DA) and glutamate release in the nucleus accumbens. This neuropharmacological profile suggests that iptakalim may be useful for treatment of nicotine dependence. Thus, we examined the effects of iptakalim in two preclinical models. First, the impact of iptakalim on the interoceptive stimulus effect of nicotine was evaluated by training rats in a discriminated goal-tracking task that included intermixed nicotine (0.4 mg/kg, SC) and saline sessions. Sucrose was intermittently presented in a response-independent manner only on nicotine sessions. On intervening test days, rats were pretreated with iptakalim (10, 30, 60 mg/kg, IP). Results revealed that iptakalim attenuated nicotine-evoked responding controlled by the nicotine stimulus in a dose-dependent manner. In a separate study, the impact of iptakalim on the reinforcing effects of nicotine was investigated by training rats to lever-press to self-administer nicotine (0.01 mg/kg/infusion) [Dosage error corrected]. Results revealed that pretreatment with iptakalim (1, 3, 6 mg/kg, IV) decreased nicotine intake (i.e., less active lever responding). Neither behavioral effect was due to a non-specific motor effect of iptakalim, nor to an ability of iptakalim to inhibit DA transporter (DAT) or serotonin transporter (SERT) function. Together, these finding support the notion that iptakalim may be an effective pharmacotherapy for increasing smoking cessation and a better understanding of its action could contribute to medication development.

Interoceptive conditioning in rats: effects of using a single training dose or a set of 5 different doses of nicotine

Interoceptive conditioning contributes to the tenacity of nicotine dependence. Previous research investigating nicotine as an interoceptive stimulus has typically employed administration of a single training dose of nicotine over an extended time. This approach has allowed for careful study of the nicotine stimulus. In humans, the nicotine stimulus is unlikely to be fixed across learning episodes. Thus, from a translational perspective, systematic variation of nicotine dose in training might better approximate interoceptive conditioning in humans. Notably, training with a class or set of discrete exteroceptive stimuli (e.g., different pictures of cars) produces interesting behavioral differences relative to training with a single stimulus. The present study sought to determine whether similar differences would occur if a set of nicotine stimuli were used in place of a single dose. To investigate this question, one group of male Sprague-Dawley rats was trained on a discriminated goal-tracking task with a set of nicotine doses (0.05, 0.125, 0.2, 0.275, and 0.35mg/kg). A second group received the standard protocol of training with a single nicotine dose (0.2mg/kg). On each nicotine session, there was intermittent access to liquid sucrose (26%) in a conditioning chamber. On intermixed saline sessions, sucrose was withheld. We examined acquisition, subsequent extinction, transfer of extinction, nicotine generalization, and mecamylamine blockade. Both groups reliably discriminated between nicotine and saline sessions, were sensitive to non-reinforcement, displayed transfer of extinction, demonstrated dose-dependent nicotine generalization, and responding was blocked by mecamylamine. There were no significant differences between the two groups. The unique nature of an interoceptive pharmacological stimulus and the challenges posed for studying the impact of training with a set of interoceptive stimuli are discussed.

Delivery of nicotine in an extract of a smokeless tobacco product reduces its reinforcement-attenuating and discriminative stimulus effects in rats

RATIONALE

Animal models of tobacco addiction rely on administration of nicotine alone or nicotine combined with isolated constituents. Models using tobacco extracts derived from tobacco products and containing a range of tobacco constituents might more accurately simulate tobacco exposure in humans.

OBJECTIVE

To compare the effects of nicotine alone and an aqueous smokeless tobacco extract in several addiction-related animal behavioral models.

METHODS

Nicotine alone and nicotine dose-equivalent concentrations of extract were compared in terms of their acute effects on intracranial self-stimulation (ICSS) thresholds, discriminative stimulus effects, and effects on locomotor activity.

RESULTS

Similar levels of nicotine and minor alkaloids were achieved using either artificial saliva or saline for extraction, supporting the clinical relevance of the saline extracts used in these studies. Extract produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, but reduced reinforcement-attenuating (ICSS threshold-increasing) effects at a high nicotine dose. In rats trained to discriminate nicotine alone from saline, intermediate extract doses did not substitute for the training dose as well as nicotine alone. Locomotor stimulant effects and nicotine distribution to brain were similar following administration of extract or nicotine alone.

CONCLUSIONS

The reinforcement-attenuating and discriminative stimulus effects of nicotine delivered in an extract of a commercial smokeless tobacco product differed from those of nicotine alone. Extracts of tobacco products may be useful for evaluating the abuse liability of those products and understanding the role of non-nicotine constituents in tobacco addiction.

Conditioned response evoked by nicotine conditioned stimulus preferentially induces c-Fos expression in medial regions of caudate-putamen

Nicotine has both unconditioned and conditioned stimulus properties. Conditioned stimulus properties of nicotine may contribute to the tenacity of nicotine addiction. The purpose of this experiment was to use neurohistochemical analysis of rapidly developing c-Fos protein to elucidate neurobiological loci involved in the processing of nicotine as an interoceptive conditioned stimulus (CS). Rats were injected (SC) in an intermixed fashion with saline or nicotine (16 sessions of each) and placed in conditioning chambers where they were given one of the three conditions depending on group assignment: (a) nicotine paired 100% of the time with intermittent access to sucrose (nicotine-CS condition), (b) nicotine and saline each paired 50% of the time with sucrose (chamber-CS condition), or (c) no sucrose US control (CS-alone condition). Rats in the nicotine-CS condition acquired the discrimination as evidenced by goal-tracking (ie, increased dipper entries before initial sucrose delivery) only on nicotine sessions. The chamber-CS condition showed goal-tracking on all sessions; no goal-tracking was seen in the CS-alone condition. On the test day, rats in each condition were challenged with saline or nicotine and later assessed for c-Fos immunoreactivity. In concordance with previous reports, nicotine induced c-Fos expression in the majority of areas tested; however, learning-dependent expression was specific to dorsomedial and ventromedial regions of caudate-putamen (dmCPu, vmCPu). Only rats in the nicotine-CS condition, when challenged with nicotine, had higher c-Fos expression in the dmCPu and vmCPu. These results suggest that medial areas of CPu involved in excitatory conditioning with an appetitive nicotine CS.

Diminished conditioned responding to the nicotine stimulus by antidepressant drugs with differing specificity for the serotonin and norepinephrine transporter

People diagnosed with depression also tend to have a co-morbid nicotine addiction. Thus, there is interest in whether medications used to treat depression alter the effects of nicotine. This study assessed whether the antidepressant drugs citalopram, imipramine, and reboxetine, with differing specificity for the serotonin and norepinephrine transporter, altered responding controlled by the conditional stimulus (CS) effects of nicotine. Rats received intermixed 20-min nicotine (0.4 mg base/kg, SC) and saline sessions. On nicotine sessions, rats had intermittent access to sucrose; no sucrose was available on saline sessions. After discrimination performance stabilized and a nicotine generalization curve (0.025-0.4 mg/kg) was established, the antidepressant drugs were assessed. In these tests, rats were pretreated with citalopram (1-17 mg/kg), imipramine (1-17 mg/kg), or reboxetine (1-30 mg/kg) before the training dose of nicotine and placement in a chamber for a 4-min extinction test. At the higher doses, all three antidepressant drugs blocked responding evoked by the nicotine CS and decreased nicotine-induced hyperactivity. When these higher doses of citalopram, imipramine, and reboxetine were tested alone (no nicotine), they decreased chamber activity and/or dipper entries. Nevertheless, all three drugs produced partial or complete blockade of the CS effects of nicotine at doses that produced no effect on dipper entries or chamber entries. This finding suggests that both neurotransmitters play a role in the CS effects of nicotine and that modifications in these systems by antidepressants may be clinically relevant.

Excitatory conditioning to the interoceptive nicotine stimulus blocks subsequent conditioning to an exteroceptive light stimulus

Previous research has shown that a nicotine conditional stimulus (CS) can compete with (i.e., overshadow) a brief light CS. Another form of competition, blocking, has not yet been examined with the nicotine CS. Groups of rats were assigned to an element training condition. For the N+ group, during each daily 2h element training session, there were ten intravenous nicotine infusions (0.03 mg/kg) followed 30s later with 4s access to sucrose. In the N- group, nicotine and sucrose presentations were explicitly unpaired. The chamber alone group (C alone) had no stimulus presentations. Element training was followed by compound training in all groups. A 30-s houselight was included during the time between the nicotine infusion and paired sucrose delivery. Non-reinforced element presentations assessed relative control of the goal tracking conditioned response (CR). The N+ group showed a higher proportion of CR control by the nicotine than the light. The opposite pattern was found in the N- and C alone groups indicating that nicotine CS controlled less of the CR than the light. Thus, excitatory conditioning with the nicotine CS blocked later conditioning to the light. This finding adds to literature examining the interaction between interoceptive drug CSs and other environmental stimuli.

Nicotine competes with a visual stimulus for control of conditioned responding

Environmental stimuli that co-occur with tobacco use come to evoke drug-related conditioned responses (CRs) that appear involved in continued use of nicotine-containing products. In rats, nicotine can serve as a conditional stimulus (CS) for non-drug unconditioned stimuli (USs), prompting the question of whether the nicotine CS can compete with, or overshadow, a non-drug environmental stimulus for control of a CR. In Experiment 1, male Sprague-Dawley rats were assigned to a group [0, 0.01, 0.03, 0.045, or 0.06 mg nicotine (base)/kg/infusion]. During each session, there were 10 intravenous infusions followed by a 30-second houselight to form a compound CS. At light offset there was 4-second access to sucrose. For Experiment 2, groups were nicotine (0.03 mg/kg/infusion) + light compound paired, nicotine + light compound unpaired, nicotine paired and light unpaired, and nicotine unpaired and light paired. Paired stimuli were presented with sucrose similar to Experiment 1. Unpaired stimuli were temporally separated from sucrose. Following acquisition, tests of nicotine and light alone were conducted by intermixing non-reinforced trails into training sessions. Nicotine dose-dependently overshadowed the light CS as shown by reduced light control of conditioned responding with higher doses. The nicotine, light, and nicotine + light compound had to be paired with sucrose to evoke a CR. These results demonstrate nicotine overshadows an exteroceptive visual stimulus. Because exteroceptive stimuli are often the focus of cue-exposure therapy, such competition may help begin to explain the marginal effectiveness of these therapies.

An examination of NMDA receptor contribution to conditioned responding evoked by the conditional stimulus effects of nicotine

RATIONALE

Research using a drug discriminated goal-tracking (DGT) task showed that the N-methyl-D: -aspartate (NMDA) channel blocker MK-801 (dizocilpine) reduced the nicotine-evoked conditioned response (CR).

OBJECTIVES

Given the unknown mechanism of the effect, Experiment 1 replicated the MK-801 results and included tests with NMDA receptor ligands. Experiments 2a and 2b tested whether MK-801 pretreatment blocked DGT via a state-dependency effect.

METHODS

In Experiment 1, adult male Sprague-Dawley rats received intermittent access to liquid sucrose following nicotine (0.4 mg base/kg); no sucrose was delivered on intermixed saline sessions. Conditioning was indicated by increased anticipatory dipper entries (goal-tracking) on nicotine compared to saline sessions. Antagonism and/or substitution tests were conducted with MK-801, phencyclidine, CGP 39551, d-CPPene (SDZ EAA 494), Ro 25,6981, L-701,324, ACPC, and NMDA. In Experiment 2a, rats received nicotine and sucrose on every session-no intermixed saline sessions without sucrose. Tests combined MK-801 or the non-competitive nicotinic acetylcholine receptor antagonist, mecamylamine with either nicotine or saline. Experiment 2b had sucrose delivered on saline sessions and no sucrose on intermixed nicotine sessions followed by MK-801 antagonism tests of the saline CS.

RESULTS

MK-801 and phencyclidine dose-dependently attenuated the CR in Experiment 1. Ro-25,6981 enhanced the CR, but did not substitute for nicotine. Other ligands showed inconsistent effects. In Experiment 2a, MK-801 pretreatment reduced goal-tracking when given before nicotine and saline test sessions; mecamylamine pretreatment had no effect. In Experiment 2b, MK-801 dose-dependently attenuated the saline-evoked CR.

CONCLUSIONS

Combined, the results suggest that MK-801 blocks discriminated goal-tracking by virtue of state-changing properties.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

Extinction with varenicline and nornicotine, but not ABT-418, weakens conditioned responding evoked by the interoceptive stimulus effects of nicotine

The interoceptive stimulus effects of nicotine acquire control over behavior. This observation, among others, suggests that the stimulus effects of nicotine are important in the development and tenacity of tobacco dependence. Despite this importance, there has been little research examining whether non-reinforced presentations (extinction) of a ligand that share stimulus effects of nicotine will weaken responding controlled by nicotine. Rats were trained to discriminate nicotine (0.4 mg/kg) from saline using a discriminated goal-tracking task in which nicotine signaled intermittent access to sucrose; sucrose was withheld on saline sessions. Experiment 1 examined substitution for nicotine by ABT-418, nornicotine, epibatidine, varenicline, or cytisine in 4-min extinction tests. Experiments 2-5 [low-dose nicotine (0.05 mg/kg), ABT-418, nornicotine, or varenicline, respectively] examined whether substitution for nicotine would persist if extinction tests were increased to 20 min and repeated daily for 6 days. Finally, generalization of this extinction back to the nicotine training stimulus was assessed. Full substitution in brief 4-min extinction tests was seen for ABT-418, nornicotine, epibatidine, varenicline, and cytisine. Low-dose nicotine, ABT-418, nornicotine, and varenicline, evoked only a partial 'nicotine-like' response in the first 20-min extinction test. With repeated extinction, only low-dose nicotine, nornicotine, and varenicline continued to substitute. Extinction with nornicotine and varenicline transferred back to nicotine as indicated by a partial conditioned response to the training stimulus. Interpretations regarding 'nicotine-like' effects of a ligand depend on the nature of the test. Understanding the processes mediating transfer of extinction learning with potential pharmacotherapies may reveal new treatment targets.

Mecamylamine, dihydro-beta-erythroidine, and dextromethorphan block conditioned responding evoked by the conditional stimulus effects of nicotine

Current smokers express the desire to quit. However, the majority find it difficult to remain abstinent. As such, research efforts continually seek to develop more effective treatment. One such area of research involves the interoceptive stimulus effects of nicotine as either a discriminative stimulus in an operant drug discrimination task, or more recently as a conditional stimulus (CS) in a discriminated goal-tracking task. The present work investigated the potential role nicotinic acetylcholine receptors play in the CS effects of nicotine (0.4mg/kg) using antagonists with differential selectivity for beta2*, alpha7*, alpha6beta2*, and alpha3beta4* receptors. Methyllycaconitine (MLA) had no effect on nicotine-evoked conditioned responding. Mecamylamine and dihydro-beta-erythroidine (DHbetaE) dose-dependently blocked responding evoked by the nicotine CS. In a time-course assessment of mecamylamine and DHbetaE, each blocked conditioned responding when given 5min before testing and still blocked conditioned responding when administered 200min before testing. Two novel bis-picolinium analogs (N, N'-(3, 3'-(dodecan-1,12-diyl)-bis-picolinium dibromide [bPiDDB], and N, N'-(decan-1,10-diyl)-bis-picolinium diiodide [bPiDI]) did not block nicotine-evoked conditioned responding. Finally, pretreatment with low dose combinations of mecamylamine, dextromethorphan, and/or bupropion was used to target alpha3beta4* receptors. No combination blocked conditioned responding evoked by the training dose of nicotine. However, a combination of mecamylamine and dextromethorphan partially blocked nicotine-evoked conditioned responding to a lower dose of nicotine (0.1mg/kg). These results indicate that beta2* and potentially alpha3beta4* nicotinic acetylcholine receptors play a role in the CS effects of nicotine and are potential targets for the development of nicotine cessation aids.

Investigation of endocannabinoid modulation of conditioned responding evoked by a nicotine CS and the Pavlovian stimulus effects of CP 55,940 in adult male rats

RATIONALE

The cannabinoid CB(1) receptor antagonist/inverse agonist rimonabant (SR 141716) has been shown to block reinforcing and rewarding effects of nicotine. Research has not investigated whether the cannabinoid system is involved in the interoceptive stimulus effects of nicotine functioning as a conditional stimulus (CS).

OBJECTIVE

We examined the effects of rimonabant and the CB(1/2) receptor agonist, CP 55,940, on responding evoked by a nicotine CS in rats. Additionally, we determined whether CP 55,940 functioned as a CS or a Pavlovian positive drug feature

MATERIALS AND METHODS

Pavlovian discrimination training involved intermixed nicotine (0.2 mg base/kg) and saline sessions with intermittent access to water only on nicotine. Antagonism tests with rimonabant (0.1-3 mg/kg) and substitution tests with CP 55,940 (0.003-0.1 mg/kg) followed. An effective dose of CP 55,940 was tested against the nicotine generalization curve. A separate group received CS training with CP 55,940 (0.01 mg/kg). Two other groups were trained using CP 55,940 (0.01 or 0.03 mg/kg) as a positive drug feature in which a brief light CS signaled access to water only on CP 55,940 sessions.

RESULTS

Rimonabant blocked nicotine-evoked responding. CP 55,940 partially substituted for nicotine and enhanced responding to lower nicotine doses. Overall, CP 55,940 did not acquire control of conditioned responding in either Pavlovian drug discrimination task.

CONCLUSIONS

The cannabinoid system was involved in the CS effects of nicotine. This finding is counter to the operant drug discrimination research with nicotine as a discriminative stimulus, warranting further research into this possible dissociation.

Nicotine-evoked conditioned responding is dependent on concentration of sucrose unconditioned stimulus

Previous studies have shown that the interoceptive nicotine conditional stimulus (CS) functions similarly to exteroceptive CSs such as lights or environments. For instance, the appetitive conditioned response (CR) evoked when nicotine is repeatedly paired with sucrose presentations (the unconditioned stimulus; US) is sensitive to changes in training dose (CS salience) and the contiguity between the CS effects and sucrose. The current study was conducted to extend this research by examining the possible role of US intensity in CR acquisition and maintenance. Rats were trained using one of four sucrose concentrations: 0, 4, 16, or 32% (w/v). On nicotine sessions (0.4 mg base/kg), rats received 36 deliveries (4s each) of their assigned concentration intermittently throughout the session; sucrose was withheld on saline sessions. In all groups, an appetitive goal-tracking CR was acquired at a similar rate. However, the asymptotic CR level varied with sucrose concentration. The magnitude of the CR was increased in rats trained with higher sucrose US concentrations. These findings are consistent with previous Pavlovian conditioning research, and extend the conditions under which the nicotine state functions as an interoceptive conditional stimulus.

Acquired appetitive responding to intravenous nicotine reflects a Pavlovian conditioned association

Recent research examining Pavlovian appetitive conditioning has extended the associative properties of nicotine from the unconditioned stimulus or reward to include the role of a conditional stimulus (CS), capable of acquiring the ability to evoke a conditioned response. To date, published research has used presession extravascular injections to examine nicotine as a contextual CS in that appetitive Pavlovian drug discrimination task. Two studies in the current research examined whether a nicotine CS can function discretely, multiple times within a session using passive iv infusions. In Experiment 1, rats readily acquired a discrimination in conditioned responding between nicotine and saline infusions when nicotine was selectively paired with sucrose presentations. In Experiment 2, rats were either trained with nicotine paired with sucrose or explicitly unpaired with sucrose. The results showed that rats trained with explicitly unpaired nicotine and sucrose did not increase dipper entries after the infusions. Nicotine was required to be reliably paired with sucrose for control of conditioned responding to develop. Implications of these findings are discussed in relation to tobacco addiction, learning theory, and pharmacology.